This invention is directed to polymerization inhibition during the manufacture, purification, handling or storage of ethylenically unsaturated monomers.
Normally, during production, including manufacture and purification, as well as during handling and storage of ethylenically unsaturated monomers, there is a need to add polymerization inhibitors such as hydroquinone (HQ), mono methyl ether of hydroquinone (MEHQ), phenothiazine (PTZ), hindered amine radical trap compounds, or a catechol, such as tertiary butyl catechol or di-tertiary butyl catechol. Additional exemplary inhibitors generally employed include phenolic compounds characterized by the presence of at least one other substituent on the benzene ring. Such other substituent serves to activate the phenolic inhibitor. Representative substituents include a C1-C4 alkoxy group such as methoxy or ethoxy, hydroxyl, sulfhydryl, amino, a C1-C9 alkyl group, phenyl, nitro, or N-linked amide, for example.
It is well known that in the absence of inhibitors, unintended polymerization, resulting in polymer fouling, can readily occur and not only impact production handling or storage of the desired product, but also result in equipment failure. Further, additional energy is required to operate the production units, such as distillation equipment, for example. Purification, particularly involving distillation equipment, is of great concern, since this equipment requires a large amount of energy to operate and operates at high temperatures. The increased temperatures increase the likelihood of polymerization occurring.
The majority of the inhibitors employed are contained in the liquid monomers. The surfaces of the equipment not in contact with the liquid containing the inhibitor provide open sites for polymerization or fouling. An example is any portion of equipment where vapors undergo condensation, since the condensate would no longer contain the inhibitors in the original monomer liquid. Hence, there is a need for protection of the equipment surfaces, which are typically metal surfaces, exposed to such monomers, as well as the monomers themselves, so as to inhibit polymer formation during manufacture, purification, handling or storage of an ethylenically unsaturated monomer.
U.S. Pat. No. 6,441,228 B2 and U.S. patent application Publication No. US 2002/0165407 A1, published Nov. 7, 2002; both of Nakahara et al., disclose a method and apparatus for producing (meth)acrylic acid, wherein the apparatus contains molybdenum so as to minimize corrosion and pitting of the surface, which in turn minimizes polymerization of the (meth)acrylic acid and associated fouling of the equipment by polymerized products. The Nakahara et al. documents disclose using for the production apparatus a metal made of a nickel chromium iron alloy having a molybdenum content of greater than 3% to 20% or a molybdenum content of 1 to 4% with 0.5% to 7% copper.
The present invention satisfies the need in the industry to inhibit polymerization of ethylenically unsaturated monomers during their manufacture, purification, handling or storage by the use of novel materials for at least a portion of the apparatus in which the monomers are in contact.